Rheology modifiers, also referred to as thickeners or viscosifiers, are ubiquitous in surfactant containing personal care cleansing formulations. Rheological properties (e.g., viscosity and flow characteristics, foamability, spreadability, and the like), aesthetic properties (e.g., clarity, sensory effects, and the like), mildness (dermal and ocular irritation mitigation), and the ability to suspend and stabilize soluble and insoluble components within a surfactant based formulation are often modified by the addition of a thickener.
Often, thickeners are introduced into surfactant formulations in solid form and mixed under conditions effective to dissolve the thickener into the liquid surfactant composition in order to effect a viscosity enhancement. Frequently, the mixing must be conducted at elevated temperatures (hot processing) in order to promote the dissolution of the solid thickener and obtain the desired viscosity improvement. Additionally, solid thickeners (e.g., Carbomer powders) are known to resist “wet-out” upon contact with the surface of an aqueous based system. Consequently, Carbomers are supplied as finely divided powders and/or must be sifted to reduce particle size which aids in dissolution by increasing the relative surface area of the particle. During processing Carbomer powders can become electrostatically charged as they are transferred in and out of containers and tend to adhere to oppositely charged surfaces including airborne dust, necessitating specialized dust extraction equipment. This means that preparation of aqueous dispersions is messy and time-consuming unless special precautions and expensive equipment is employed. Formulators of compositions containing thickened surfactant constituents desire the ability to formulate their products at ambient temperatures (cold processing). Accordingly, formulators desire thickeners, which can be introduced to the liquid surfactant compositions in liquid form rather than as a solid. This provides the formulator with a greater degree of precision in introducing the thickener to the liquid surfactant composition, allows the ability to formulate products at ambient temperatures (cold processing), and better facilitates automated processing without the need for special safety and handling equipment.
One important class of liquid rheology modifier commonly employed to thicken aqueous based surfactant containing formulations is the alkali-swellable or alkali-soluble emulsion (ASE) polymers. ASE polymers are linear or crosslinked copolymers that are synthesized from (meth) acrylic acid and alkyl acrylates. The crosslinked polymers immediately thicken upon neutralization with an inorganic or an organic base. As liquid emulsions, ASE polymers are easily processed and formulated into liquid surfactant containing formulations by the product formulator. Examples of ASE polymer thickened surfactant based formulations are set forth in U.S. Pat. No. 6,635,702; International Published Application No. WO 01/19946; and European Patent No. 1 690 878 B1, which disclose the use of a polymeric thickener for aqueous compositions containing surfactants. Although these thickeners offer a good viscosity, suspension and clarity properties in surfactant containing formulations at pH values near neutral (pH≧6.0), they become hazy at acidic pH ranges, resulting in poor clarity.
Microbial contamination from bacteria, yeast, and/or fungus in cosmetics, toiletries and personal care products is very common and has been of great concern to the industry for many years. Present day surfactant containing products are typically formulated with a preservative to protect the product from decay, discoloration, or spoilage and to ensure that the product is safe for topical application to the skin, scalp, and hair in humans and animals. Three classes of preservative compounds that are commonly used in surfactant containing products are the formaldehyde donors such as diazolinyl urea, imidazolinyl urea, and DMDM Hydantoin; the halogenated compounds including 2,4-dichlorobenzyl-alcohol, Chloroxylenol (4-chloro-3,5-dimethyl-phenol), Bronopol (2-bromo-2-nitropropane-1,3-diol), and iodopropynyl butyl carbamate; and the paraben compounds including methyl-paraben, ethyl-paraben, propyl-paraben, butyl-paraben, isopropyl-paraben, and benzyl-paraben.
While these preservatives have been successfully utilized in personal care products for many years, there are recent concerns by the scientific community and the public that some of these compounds may constitute health hazards. Accordingly there is an interest in replacing the above-mentioned compounds in surfactant containing products that are topically applied to or come into contact with human skin, scalp or hair while maintaining good antimicrobial efficacy, mildness, and do not raise safety concerns.
Organic acids (e.g., sorbic, citric and benzoic), such as those used as preservatives in the food industry, have been increasingly looked at as the ideal replacement for foregoing preservative systems in surfactant containing formulations. The antimicrobial activity of the organic acids is connected to the associated or protonated species of the acid molecule. As the pH of an organic acid containing formulation increases, dissociation of the proton occurs forming acid salts. The dissociated form of the organic acids (acid salts) have no antimicrobial activity when used alone, effectively limiting the use of organic based acids to pH values below 6 (Weber, K. 2005. New alternatives to paraben-based preservative blends. Cosmetics & Toiletries 120(1): 57-62).
The literature has also suggested that formulating products in the natural pH range (between about 3-5) 1) reduces the amount of preservative required in a product by enhancing preservative efficacy, 2) stabilizes and increases the effectiveness of many cosmetic active ingredients, 3) is beneficial to the repair and maintenance of skin barrier tissue, and 4) supports the natural skin flora to the exclusion of over-colonization by deleterious microorganisms (Wiechers, J. W. 2008. Formulating at pH 4-5: How lower pH benefits the skin and formulations. Cosmetics & Toiletries 123(12): 61-70).
As the industry desires new thickened surfactant based products that are formulated in the acidic pH range, there is a developing need for a rheology modifier that, when used in combination with a surfactant, provides a high clarity formulation under acidic pH conditions while maintaining a good viscosity/rheology profile, suspension (yield value), and enhanced aesthetics.